Propylene random copolymer usually refers to a polymer obtained by randomly copolymerizing a small amount, usually not more than 15% by weight, of α-olefin monomer, for example, commonly used ethylene or butene-1, onto a polypropylene molecular chain. Since the co-monomers destroy the regularity of the arrangement of the propylene units in the molecular chain, the melting point and the degree of crystallinity of the polypropylene resin are declined. The relatively low melting point enables the polypropylene resin to be used for manufacturing a more readily heat-sealable film and the like, while the relatively low degree of crystallinity results in improved transparency of the material. Although co-monomers are often copolymerized in a larger amount in order to achieve higher transparency, the excessively increasing in the amount of co-monomers may unduly reduce the degree of crystallinity of the material, thereby reducing the rigidity and heat resistance of the material, and more important also increase the content of the non-crystalline components (the soluble contents) in the material, so that they tend to migrate onto the surface of articles and thus affect negatively the processing and application of the product. In particular, in case that the product is used for packaging greasy foods, a part of the non-crystalline components may gradually migrate to the food, and contaminate it. In addition, reducing the melting point and the degree of crystallinity usually also render heat resistance of the material deteriorated. If such a material is used in food containers, the deteriorated heat resistance makes it disadvantageous to the microwave heating. The use of co-monomers in a unduly larger amount may also bring difficulties to the polymerization process, for example, the occurrence of kettle-sticking, caking and hard removal of residual monomers, in particular when the co-monomer is butene-1 having higher boiling point or α-olefins having more carbon atoms.
In the prior art, butene-1 and propylene are often used to carry out random copolymerization, with the expectation that the resultant propylene-butene-1 random copolymer has a relatively low migrants content. However, the random copolymer still has drawbacks in giving consideration to both transparency and heat resistance. An existing solution is to combine with an appropriate amount of nucleating agent that can improve the transparency. Although this can, to some extent, improve the transparency and heat resistance of the copolymer material, the result is still not satisfactory in terms of the heat resistance and the migrants content.